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Report 8

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Cherutich

FIGURĘ 8: Top drive set-up showing circulation while running in hole

FIGURĘ 9: Rotary table set-up cannot circulate while running in hole

formation temperaturę of 350°C. In the WDla well, the deepest depth the O-ring seal survived was 3,45 lm with a static formation temperaturę of 450°C and 5 bits had an average of 50 drill hours without the O-ring seal failing. Judging from these two examples, the bit life of a top drive system is 3-6 times longer than that of the convectional system under the same drilling conditions.

4.4 Ratę of penetration

Due to the worldwide increase in petroleum prices and materiał costs, there is a need to lower the well cost, e.g. by increasing the ratę of penetration (see Figurę 7) and minimising as much as possible the non-drilling costs. The ratę of penetration depends mainly on the weight on the bit (WOB), the revolutions per minutes (RPM), the cutting condition of the bit and the formation hardness. There is a rule of thumb that the w eight on the bit should be about 1 ton per 1" of the diameter of the drill bit but the rotation applied depends mainly on the hardness of the rock being drilled. The weight on the bit comes from the drill collars. The cutting condition of the drill bit inserts is very important, for example if the teeth are duli it overrules all other factors. Also the circulation fluid is important in improving the ratę of penetration; the drill cuttings need to be removed in order to have a clean surface for the next teeth to cut the new surface. This is common while drilling on a medium hard formation where the ratę of penetration is high, thus generating a high volume of cuttings that need to be removed. Because the top drive has a better bit life management than the convectional rig type, a higher ratę of penetration can be achieved. Underbalanced drilling sometimes improves the ratę of penetration by as much as 50% as compared to overbalanced drilling.

There exists a relationship between the bit life and its cost. The bits with a joumal bearing, tungsten insert are at the higher end of the cost spectrum while milled tooth bits with bali bearings are at the lower end.

4.5 Probability of getting stuck and the fishing process

Drilling is an industry where everyone knows that time is money. To lower drilling costs one should design drilling programmes that can minimise any non-productive drilling operation time. One of the